1. Field of the Invention
The present invention is related to the cold rolled steel sheet for making a shadow mask of a cathode ray tube of a color Braun tube for selecting colors and a manufacturing method therefor. In more detail, the present invention is related to a cold rolled steel sheet for making a shadow mask and a manufacturing method therefor, in which the cold rolled steel sheet is manufactured by one-step cold rolling and low-temperature annealing by properly controlling the chemical composition and manufacturing process, rather than by two-step cold rolling and open coil annealing so that the cold rolled steel sheet has the equivalent etchability and formability as those of the conventional products which are required for the material for a shadow mask while reducing the production steps and manufacturing cost significantly.
2. Description of the Prior Art
Conventionally, the cold rolled steel sheet for a shadow mask has been manufactured by going through steel manufacture and hot rolling which are the usual manufacturing process of the steel sheet, open coil annealing after the first cold rolling as shown in FIG. 1(a) in order to remove solute carbons which have negative effects on formability and magnetic properties, and the second cold rolling process. Therefore, besides highly expensive open coil annealing (OCA) facilities and usual cold rolling facilities, separate second rolling facilities (DCR-mill, double cold reduction) for the second cold rolling have been required for the manufacture of the cold rolled steel sheet for a shadow mask. For this reason, the manufacturing cost for the cold rolled steel sheet for a shadow mask is five times higher than that of the cold rolled steel sheet which is produced according to the conventional processes. However, contrary to these processes, in reality, it does not seem that the manufacturing method according to the one-step rolling, which does not require open coil annealing and the second rolling, and other conventional annealing facilities have been established.
A shadow mask (10) is, as shown in FIG. 2, a part attached to inside of a Braun tube (1) which is vacuum and is composed of very minute holes (11). Minute holes are parts that reproduce the final color by properly selecting electron beams, which are responsible for red, blue, and green colors coming from an electron gun (2).
Therefore, requirements for a cold rolled steel sheet for a shadow mask would be the etchability of holes, magnetic property, press formability, black oxide film adherence, maintenance of degree of vacuum, and others. In order to satisfy such composite requirements, a cold rolled steel sheet for a shadow mask has to have cleanness without impurities, no coarse precipitates, strictly controlled thickness, and superior shape.
Among the processes shown in FIG. 1(b), hundreds of thousands of minute holes having a diameter of about 0.2 mm are made during the etching process according to the photo-etching technique by using the ferric chloride solution. At this time, the shapes of etched holes have to be perfect, and homogeneity has to be secured. If the shapes of holes are not homogeneous, blotting of colors occurs in the color Braun tube. Therefore, having superior etchability is the basic requirement for a cold rolled steel sheet for a shadow mask.
The shadow mask which has been etched goes through the second annealing in order to facilitate the forming process which is the next process. Proper ductility is secured for the material through the annealing process and very superior formability is required in order to have a necessary bending radius suitable for a Braun tube. It is because forming characteristics for a sheet having minute holes appear to be different from those for a sheet having no minute holes. If the deformation around holes is not homogeneous, the shapes of holes may be changed during the forming process, resulting in the blotting of colors. Carbons in the steel assume a very important role in order to secure a superior formability.
The most important element determining strength of the ordinary steel sheet is carbon. Steels are divided into the high carbon steel, medium carbon steel, low carbon steel, and extremely low carbon steel according to the amount of carbon content. Low carbon steel and extremely low carbon steel are generally used for a cold rolled steel sheet for a shadow mask. Carbons in the steel take the form of a Fe.sub.3 C compound, i.e., a carbide, or a solute carbon as an atom. Carbides are precipitated along the grain boundary, and solute carbons can not be observed even through a microscope since they locate at the interstitial sites among Fe atoms as shown in FIG. 3. Also, carbon atoms are very small compared to Fe atoms, and therefore, they cause the strain aging phenomenon by interacting with dislocations which are the plastic deformation mechanism of steel plates. In other words, deformation of a steel sheet by the external force is due to the movement of dislocations in the steel sheet, which is hindered by solute carbons. Accordingly, dislocations move as shown in FIG. 4(b) and the movement is not smooth as shown in FIG. 4(a) due to the hindrance by solute carbons, and this movement causes the yield point elongation during tensile tests as shown in FIG. 5.
If such yield point elongation still exists after the second annealing in a cold rolled steel sheet for a shadow mask, the steel sheet of high quality may not be obtained since the hardness of the material is increased, shape fixability is lowered, and stretcher strain, in which the shapes of holes are changed to be non-homogeneous according to the yield point elongation, occurs. Therefore, reducing the amount of solute carbons in the steel during the conventional open coil annealing is essential for decreasing the hardness and eliminating the yield point elongation in the manufacture of a cold rolled steel sheet for a shadow mask. The titanium-added extremely low carbon steel does not show the yield point elongation because all the carbon is precipitated as TiC. However, it has been definitely disadvantageous in that its manufacturing process is somewhat complicated, there are problems of clogging of nozzle during continuous casting processes and other problems, and particularly, the magnetic property is extremely lowered due to fine precipitates formed. Further, the shadow mask formed as desired is subject to the blackening processing. This is to prevent oxidation or blue-coloring of the shadow mask as well as to absorb or discharge heat in the Braun tube effectively.
In the meantime, inside of a Braun tube has to be shielded from the external magnetic field in order to allow electron beams to progress correctly to the desired direction. For this reason, a shadow mask steel sheet has to have superior magnetic property, and require for the coercive force of less than 1.3 Oe generally. Also, maintenance of the degree of vacuum inside of a Braun tube has to be secured so that no gas is discharged from inside of the steel sheet as the time goes by. It is because the life of the electron gun is shortened, and it can not perform its function as a Braun tube if the degree of vacuum is lowered.
The present invention is, therefore, to solve the problem of very high manufacturing cost of a cold rolled steel sheet for a shadow mask manufactured by the current two-step cold rolling and open coil annealing processes and to manufacture a cold rolled steel sheet of a shadow mask by employing the one-step cold rolling and low-temperature annealing processes while satisfying the above-described requirements through metallurgical researches and experiments and based on the results of such researches and experiments.